It's important to remember that Microsoft doesn't really care where the money comes from, as long as the money comes in.
If all of their games suddenly sold majority-PC instead of console, there's absolutely no doubt that they'd drop Xbox without a second thought.
If all of their games suddenly sold majority-console instead of PC, they'd drop PC support.
Or if their games stopped profiting at all, they'd instantly drop that entire concept and move on to something else.
This means that any sort of talk about 'quality' is missing the point. Microsoft only needs just enough quality to make people buy the game. Any more than that is a waste of resources.
(Note: Most companies act this way, so I'm not bashing Microsoft, or anybody else. It's just business.)
Yes, Microsoft would unquestionably sell more consoles if they never released games on PCs.
But the sale of consoles would - almost certainly - not offset the sale of PC games.
Thus, they release on multiple platforms, as is the case for most companies.